Collective nouns are used to describe groups of things, like a flock of birds or a herd of elephants. Since kidnap is an action, not a group of things, it doesn't have a collective noun.
You could use phrases like:
* A series of kidnappings
* Multiple instances of kidnapping
* A wave of kidnappings
* A string of kidnappings
These phrases convey the idea of multiple kidnappings occurring together.